A long sought-after goal of photographic optical design is to provide a large aperture objective having an f/number of about f/1.5 or smaller that is well corrected for all aberrations over a large flat field and over a large spectral range. In addition, a goal for objectives used in cinematography is that they be corrected for “breathing.” Breathing is defined as a change in chief ray angle in object space as the lens is focused, and it causes objects to move radially in the image frame as they go in and out of focus.
The Petzval design form is capable of being well corrected at a large aperture over a small field of view, and has been used for many decades in applications such as cinema projection lenses, heads-up displays, and microscope objectives. Although the Petzval form is too limited for use as a general-purpose photographic or cinematographic lens, it provides important insights about large aperture lens design. In particular, a Petzval lens achieves excellent correction at large apertures by minimizing the power of the negative lens elements located far from the image plane. Petzval objectives often employ a negative field flattener located fairly close to the image plane. As a result, the positive lens elements can be made from low-index fluor crown glass or calcium fluoride crystal if desired in order to reduce or eliminate the secondary spectrum.
Examples of large aperture Petzval lenses include U.S. Pat. Nos. 2,649,021; 3,255,664; and 4,329,024.
The double-Gauss design form is very widely used for high-speed objectives that must cover a relatively wide field of view. However, double-Gauss designs rely on the use of positive elements made of high index crown glass in order to flatten the field, and as a result it is difficult or impossible to correct the secondary spectrum. Double-Gauss designs also have a strong tendency to suffer from oblique spherical aberration that severely limits off-axis performance at wide apertures. Vignetting is normally used to control the aberrated tangential rays, so high-speed double-Gauss designs have a characteristic strong illumination fall off coupled with a large amount of residual sagittal oblique spherical aberration. The oblique spherical aberration can be corrected to a large extent by relaxing the requirement for a large working distance and/or by introducing one or more aspheric surfaces into the design. Examples of high-speed double-Gauss designs include U.S. Pat. Nos. 2,012,822; 3,504,961; and 4,394,095.
Triplet derivatives, including the Sonnar and Ernostar design forms, have also been widely used in the past for high-speed objectives with small to moderate fields of view. These designs tend to have many of the strengths and shortcomings of the double-Gauss type designs, but are generally more suited to narrower fields of view. Examples include U.S. Pat. Nos. 1,975,678; 2,310,502; and 3,994,576. An interesting sub grouping of triplet derivatives designed mainly for television cameras is moderately well corrected for extremely large apertures of about f/0.7 and incorporate a meniscus doublet lens group with a very strong concave surface near the image plane. This meniscus group serves to flatten the field. These designs also have characteristics similar to Petzval lenses, particularly the minimization of negative power in large elements located far from the image plane. Examples of this type include U.S. Pat. Nos. 2,978,957; 3,300,267; 3,445,154; 3,454,326; and 3,586,420.
The reverse telephoto design form is useful for achieving both a large aperture and a wide field of view. However, the image quality for high-speed wide-angle examples is generally mediocre, and these lenses need to be stopped down substantially to achieve good results. An exception is found in microfilm objectives, such as U.S. Pat. Nos. 3,817,602 and 4,310,223; and microscope objectives such as U.S. Pat. No. 5,920,432. However in these cases a high image quality is achieved at the expense of image size. Examples of high-speed wide-angle reverse telephoto designs include U.S. Pat. Nos. 3,992,085; 4,025,169; 4,095,873; 4,136,931; and 5,315,441.
Despite the above efforts, virtually all high-speed photographic lenses designed to date are compromised by the need to balance large aberrations against each other. Most often these designs are notably soft in the outer sub-group of the image field, and must be stopped far down to achieve good performance. Accordingly, there is a need for high-speed optical systems that are both extremely well corrected and reasonably compact when scaled to an image diagonal of about 28 mm. In addition, for cinematographic applications, there is a need for such optical systems to be well corrected for breathing.